Monsoon winds carry rain-laden clouds through China every summer, providing nearly 80 percent of the annual precipitation in some parts of the country.

When the winds are weak, little to no rain reaches large expanses of China, often plunging those areas into drought.

The new study "is a brilliant analysis of the problematic coincidence of abrupt climate changes and changes in political organization," said Harvey Weiss, an archaeologist at Yale University who was not involved in the study.

"Really Surprised"

In the Wanxiang cave in north-central China, researchers discovered a record of monsoon activity preserved in a stalagmite—a rock formed by mineral-rich waters dripping onto the cave floor year after year.

The rock had been growing continuously for 1,800 years, from A.D. 190 to 2003.

Like trees, stalagmites have annual growth rings that can provide clues about local environmental conditions for a particular year.

The team measured the amount of oxygen-18—a rare form of "heavy" oxygen—in the stalagmite growth rings. Growth rings with large amounts of oxygen-18 indicate years of weak summer monsoons and less rains.

Comparing the stalagmite record with Chinese history, the researchers found that a period of strong monsoons was associated with the "golden age" of the Northern Song dynasty.

During that time, improved rice yields allowed the population to increase from 60 million to as many as 120 million.